Given a geometry and a plus code length (2, 4, 6, 8, 10, 11, 12, 13, 14, or 15), returns a grid of all rectangles represented by plus codes of that length that intersect the geometry.

pluscode_grid.sql

-- https://dbfiddle.uk/3CgLervE

-- This uses function pluscode_decode, implemented here:
-- https://github.com/google/open-location-code/blob/main/plpgsql/pluscode_functions.sql

CREATE OR REPLACE FUNCTION pluscode_grid(original_geometry geometry, pluscode_length integer)
-- Given a geometry and a plus code length (2, 4, 6, 8, 10, 11, 12, 13, 14, or 15), returns a
-- grid of all rectangles represented by plus codes of that length that intersect the geometry.
	RETURNS TABLE (pluscode text, geom geometry)
	LANGUAGE SQL
	IMMUTABLE
	RETURNS NULL ON NULL INPUT
	PARALLEL SAFE
BEGIN ATOMIC
	WITH RECURSIVE constants AS (
		SELECT
			'23456789CFGHJMPQRVWX' AS code_alphabet,
			ST_Transform($1, 4326) AS wgs84_geom
	), grid0 AS (
			-- Base case: Generate the 2-character plus codes
			SELECT
				geom,
				pluscode,
				2 AS code_length
			FROM constants AS c,
				generate_series(1, 9) AS i,
				generate_series(1, 18) AS j,
				concat(
					substr(c.code_alphabet, i, 1),
					substr(c.code_alphabet, j, 1),
					'000000+'
				) AS pluscode,
				pluscode_decode(pluscode),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS geom
			WHERE ST_Intersects(wgs84_geom, geom)
		UNION ALL
			-- Recursive step for lengths 4, 6, 8, 10: Add two characters at a time
			SELECT
				pluscode_geom,
				code,
				g.code_length + 2
			FROM grid0 AS g,
				constants AS c,
				generate_series(1, 20) AS i,
				generate_series(1, 20) AS j,
				concat(
					CASE g.code_length WHEN 8 THEN g.pluscode ELSE left(g.pluscode, g.code_length) END,
					substr(c.code_alphabet, i, 1),
					substr(c.code_alphabet, j, 1),
					right('0000+', greatest(7 - g.code_length, 0))
				) AS code,
				pluscode_decode(code),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS pluscode_geom
			WHERE g.code_length < 10
				AND g.code_length < $2
				AND ST_Intersects(wgs84_geom, pluscode_geom)
	), grid AS (
			SELECT * FROM grid0
		UNION ALL
			-- Recursive step for lengths 11 to 15: Add one character at a time
			SELECT
				pluscode_geom,
				code,
				g.code_length + 1
			FROM grid AS g,
				constants AS c,
				generate_series(1, 20) AS i,
				concat(
					g.pluscode,
					substr(c.code_alphabet, i, 1)
				) AS code,
				pluscode_decode(code),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS pluscode_geom
			WHERE g.code_length >= 10
				AND g.code_length < $2
				AND ST_Intersects(wgs84_geom, pluscode_geom)
	)
	SELECT
		pluscode,
		ST_Transform(geom, ST_SRID($1)) AS geom
	FROM grid
	WHERE code_length = $2;
END;

pluscode_grid_compact.sql

-- https://dbfiddle.uk/3CgLervE

-- This uses function pluscode_decode, implemented here:
-- https://github.com/google/open-location-code/blob/main/plpgsql/pluscode_functions.sql

CREATE OR REPLACE FUNCTION pluscode_grid_compact(original_geometry geometry, pluscode_length integer)
-- Given a geometry and a plus code length, returns a set of non-overlapping rectangles represented
-- by plus codes of that length or lower which, together, contain the geometry, aiming to minimize
-- both the number of rectangles inside the geometry and the area outside it.
	RETURNS TABLE (pluscode text, geom geometry)
	LANGUAGE SQL
	IMMUTABLE
	RETURNS NULL ON NULL INPUT
	PARALLEL SAFE
BEGIN ATOMIC
	WITH RECURSIVE constants AS (
		SELECT
			'23456789CFGHJMPQRVWX' AS code_alphabet,
			ST_Transform($1, 4326) AS wgs84_geom
	), grid0 AS (
			-- Base case: Generate the 2-character plus codes
			SELECT
				geom,
				pluscode,
				2 AS code_length,
				FALSE AS contained
			FROM constants AS c,
				generate_series(1, 9) AS i,
				generate_series(1, 18) AS j,
				concat(
					substr(c.code_alphabet, i, 1),
					substr(c.code_alphabet, j, 1),
					'000000+'
				) AS pluscode,
				pluscode_decode(pluscode),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS geom
			WHERE ST_Intersects(wgs84_geom, geom)
		UNION ALL
			-- Recursive step for lengths 4, 6, 8, 10: Add two characters at a time
			SELECT
				pluscode_geom,
				code,
				g.code_length + 2,
				ST_Contains(wgs84_geom, pluscode_geom)
			FROM grid0 AS g,
				constants AS c,
				generate_series(1, 20) AS i,
				generate_series(1, 20) AS j,
				concat(
					CASE g.code_length WHEN 8 THEN g.pluscode ELSE left(g.pluscode, g.code_length) END,
					substr(c.code_alphabet, i, 1),
					substr(c.code_alphabet, j, 1),
					right('0000+', greatest(7 - g.code_length, 0))
				) AS code,
				pluscode_decode(code),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS pluscode_geom
			WHERE g.code_length < 10
				AND g.code_length < $2
				AND NOT g.contained
				AND ST_Intersects(wgs84_geom, pluscode_geom)
	), grid AS (
			SELECT * FROM grid0
		UNION ALL
			-- Recursive step for lengths 11 to 15: Add one character at a time
			SELECT
				pluscode_geom,
				code,
				g.code_length + 1,
				ST_Contains(wgs84_geom, pluscode_geom)
			FROM grid AS g,
				constants AS c,
				generate_series(1, 20) AS i,
				concat(
					g.pluscode,
					substr(c.code_alphabet, i, 1)
				) AS code,
				pluscode_decode(code),
				ST_MakeEnvelope(lng_lo, lat_lo, lng_hi, lat_hi, 4326) AS pluscode_geom
			WHERE g.code_length >= 10
				AND g.code_length < $2
				AND NOT g.contained
				AND ST_Intersects(wgs84_geom, pluscode_geom)
	)
	SELECT
		pluscode,
		ST_Transform(geom, ST_SRID($1)) AS geom
	FROM grid
	WHERE contained OR code_length = $2;
END;